Interpretive Summary: Barley yellow dwarf (BYD) disease can lead to significant yield losses in wheat, barley, corn and oats. The aphid Schizaphis graminum is an important vector of the viruses causing BYD, but populations of the aphid differ in their ability to transmit the viruses. This paper investigates the reasons for the differences in transmission and shows that virus transmission is genetically controlled in the aphid. There are multiple aphid genes involved in virus transmission and these genes are inherited in a simple manner. Interestingly, the transmission of the different viruses causing BYD are regulated by different, but overlapping sets of aphid genes. The transmission of all of the viruses is regulated by a small number of major genes, and transmission of each individual virus is influenced by multiple minor genes. It should be relatively easy to identify the major genes in the aphid which control virus transmission. Identification of the genes involved will aid in the design of novel control strategies that interrupt virus transmission.

Technical Abstract:
The aphid Schizaphis graminum is an important vector of the viruses that cause barley yellow dwarf disease. We studied the genetic architecture of virus transmission by crossing a vector and a nonvector genotype of S. graminum. F1 and F2 hybrids were generated, and a modified line-cross biometrical analysis was performed on transmission phenotype of two of the viruses that cause barley yellow dwarf: Cereal yellow dwarf virus (CYDV)-RPV and Barley yellow dwarf virus (BYDV)-SGV. Our aims were to (1) determine to what extent differences in transmission ability between vectors and nonvectors is due to net additive or non-additive gene action, (2) estimate the number of loci that determine transmission ability, and (3) examine the nature of genetic correlations between transmission of CYDV-RPV and BYDV-SGV. Only additive effects contributed significantly to divergence in transmission of both CYDV-RPV and BYDV-SGV. For each luteovirus, Castle-Wright’s estimator for the number of effective factors segregating for transmission phenotype was less than one. Transmission of CYDV-RPV and BYDV-SGV was significantly correlated in the F2 generation, suggesting that there is a partial genetic overlap for transmission of these luteoviruses. Yet, 63% of the F2 genotypes transmitted CYDV-RPV and BYDV-SGV at significantly different rates. Our data suggest that in S. graminum, the transmission efficiency of both CYDV-RPV and BYDV-SGV is regulated by a major gene or set of tightly linked genes, and the transmission efficiency of each virus is influenced by a unique set of minor genes.